In addition to storing fat deposits, adipocytes secrete several cytokines important in regulating lipid and glucose metabolism in mammals. These so called “adipokines” include adiponectin, adipsin, leptin, and vaspin. In the literature, adiponectin has also been called GBP28, ApM1, ACRP30, AdipoQ, and OBG3. Unlike other adipokines, however, adiponectin serum levels are inversely correlated with obesity, insulin resistance and ischemic heart disease (Goldstein and Scalia (2004) The Journal of Clinical Endocrinology and Metabolism 89:2563-8, entirely incorporated by reference). While serum levels of adiponectin in normal humans typically range from 2 to 10 μg/mL, levels of circulating adiponectin are dramatically reduced in obese or diabetic individuals. Accordingly, adiponectin replacement therapy has been suggested as a possible treatment to reverse insulin resistance in type II diabetics and to ameliorate vascular atherosclerosis in at-risk cardiac patients, and decrease TNFα levels.
Adiponectin treatment has been shown to mobilize glucose uptake, increase fatty acid clearance from the circulation, and induce insulin sensitivity in both normal and insulin resistant tissues (Wu et al. (2003) Diabetes 52:1355-63; Fruebis et al. (2001) PNAS 98:2005-10; Berg et al. (2002) TRENDS in Endocrinology and Metabolism 13:84-9; all entirely incorporated by reference). Additional studies have shown that adiponectin has both cardioprotective and anti-inflammatory properties (Shimada et al (2004) Clinica. Chemica. Acta. 344:1-12; Hug and Lodish (2005) Current Opinion in Pharmacology 5:129-34, all entirely incorporated by reference). Adiponectin activity is mediated at least in part by it's stimulatory effects on the phosphorylation and subsequent activation of 5′-AMP-activated protein kinase (AMPK), the AMPK downstream substrate acetyl coenzyme A carboxylase (ACC) (Yamauchi et al. (2002) Nature Medicine 8:1288-95, entirely incorporated by reference), and also the pPAR family of steroid hormone receptors (Yamauchi et al. (200) Journal of Biological Chemistry 278:2461-8, entirely incorporated by reference). Recent studies show that adiponectin can interact with and alter the activity of several growth factors including platelet derived growth factor BB (PDGF-BB), heparin-binding epidermal growth factor-like growth factor (HB-EGF), and basic fibroblast growth factor (basic FGF) (Wang et al. (2005) Journal of Biological Chemistry 280:18341-7, entirely incorporated by reference).
Adiponectin is a 30 kD glycoprotein consisting of an N-terminal collagen-like domain, approximately residues 1-100, containing multiple G-X-X-G repeats and a C-terminal domain, approximately residues 108-244, structurally resembling the globular portions of the C1Q and TNF superfamily members. At least two proteolytic cleavage sites are located between the collagen and C1Q-like domains. Both full length and proteolytically cleaved forms are found in human serum. Globular head domain cleavage fragments of adiponectin (“globular” adiponectin or gAd) form trimeric structures, while full length adiponectin is capable of forming trimers, hexamers, and additional higher order oligomers. Mutation of the cysteine residue located in the collagen domain (conserved in all known mammalian adiponectin) abolishes hexamer and high-order oligomer formation.
Homologous proteins to adiponectin include, but are not limited to, mouse C1q/TNF-α Related Proteins 1 (CTRP1), CTRP2, CTRP3, CTRP4, CTRP5, CTRP6 and CTRP7. At least one of these proteins (CTRP2) is able to stimulate fatty acid oxidation in skeletal muscle, thus resembling the functional properties of adiponectin (Wong et al. (2004) Proc. Natl. Acad. Sci. 101:10302-7, entirely incorporated by reference).
Several adiponectin polymorphisms have been discovered within particular human populations. The severity of the phenotype depends on the position of the mutation. For example, the G84R, G90S, Y111H, and I164T mutations cause diabetes and hypoadiponectinemia as a result of a failure to form higher order oligomers that are likely important in regulating insulin sensitivity by the liver (Waki et al. (2003) J. Biol. Chem. 278:40352-63, entirely incorporated by reference). Functionally benign polymorphisms include R221S and H241P.
Based on their amino acid sequences, both known adiponectin receptors (AdipoR1 and AdipoR2) are predicted to contain seven transmembrane alpha helices but are not related to G-coupled protein receptors (Yamauchi et al. (2003) Nature 423:762-9, entirely incorporated by reference). Although AdipoR1 and AdipoR2 are homologous (>67% identity), their relative affinities to adiponectin and gAd differ. AdipoR1, expressed predominantly in skeletal muscle, binds to gAd with higher affinity than adiponectin, while AdipoR2, expressed predominantly in liver, binds preferentially to adiponectin. In vivo results in mice suggest that trimeric gAd may be more effective at reducing weight and improving insulin sensitivity than hexameric and higher order oligomeric forms of adiponectin (Yamauchi et al. (2001) Nature Medicine 7:941-6, entirely incorporated by reference).
While full-length adiponectin and gAd are very interesting pharmaceutical candidates, both full-length adiponectin and adiponectin fragments of naturally occurring adiponectin, in all known species, are very insoluble. In order to study the effects of adiponectin on any species larger than a mouse, variants of adiponectin with increased solubility are needed. Additionally, in order to produce pharmaceutically relevant quantities of full-length adiponectin or gAd, variants of adiponectin with very improved solubility are needed.